Conventional fire and smoke detection methods and apparatus generally include the use of well-known smoke and heat detectors, such as ionization smoke detectors and photooptical smoke detectors. These devices can be used as independent detector systems, such as those typically found in home use, or as peripheral devices reporting alarm conditions to a centralized system as is commonly used in larger buildings and in industrial use.
Whether these devices are used as stand-alone systems or peripheral devices, the principle of their operation is generally the same. For example, a light-scattering type photooptical detector generally comprises a light-emitting source, such as a light-emitting diode (LED), and a light sensor, such as a photo diode, contained in a substantially light proof sample chamber having low reflectance walls. Light from the light-emitting source is reflected off the low reflectance walls to the light sensor, which is out of the direct path of light. Air surrounding the photooptical detector passes generally freely in and out of the sample chamber. When ambient air is relatively free from fire or combustion products, such as smoke, only a relatively small amount of light from the LED is reflected off the chamber walls to be detected by the light sensor. This low light receiving condition is the normal or no-alarm state in the photooptical detector.
As the amount of combustion products increases, the amount of light reflected or scattered by the combustion products increases. The increased light scattering generally increases the amount of light reaching the light sensor proportionally. This phenomenon generally correlates to percent obscuration per foot. Simply put, percent obscuration per foot is a measurement of the reduction in visibility the human eye would see in a room containing combustion products.
FIG. 1 is a graph 10 illustrating the typical operation of an existing alarm. The amount of light detected by the light sensor may be represented as a voltage output, for example in the range of 0 volts and 5 volts. The curve 12 represents the detector voltage output as it varies in time due to circumstances presented for exemplary purposes. As the amount of light detected by the light detector increases due to increased combustion products, the voltage output generally increases. Conventional ionization detectors also output increasing voltage as the smoke condition rises. When, at 16, the detector voltage output reaches a predetermined alarm threshold 14, an alarm condition is indicated by audible, visual or other indications for appropriate investigation or evacuation of the alarm area.
Many home alarm detectors automatically reset at 18 when the measured parameter (the detector voltage output) again falls below the alarm threshold 14. A small amount of hysteresis (not shown) may be provided to prevent the alarm from needlessly and annoyingly transitioning back and forth between alarm and non-alarm states when the measured parameter hovers for a time at or near the alarm threshold 14.
In another typical fire alarm operation, the alarm does not automatically reset itself, and emergency personnel must reset the fire alarm system after investigating the source of an alarm, for example, at 20. For an alarm reset to take place, the heat and/or smoke sensor(s) must be at a reading (temperature or “% smoke obscuration”) lower than the alarm threshold 14.
For example, a 135° F. heat sensor will transition into an alarm state when the ambient temperature reaches 135° F. In the present art, a fire alarm system allows the system to reset to a normal (non-alarm) state as long as the measured parameter, at the time the reset key is pressed, is below the alarm threshold 14. The same holds true for smoke sensors, which are rated in “% obscuration per foot.” As long as the sensor reading at reset is below the alarm threshold 14, a fire alarm control panel will perform a reset and indicate a normal condition.